The invention relates to a method of adapting the engine to the richness of the fuel, and especially to the octane number of this fuel. This invention notably relates to controlled-ignition engines the ignition advance of which can be controlled electronically. More particularly, the invention relates to a method of adaptation by decrementing the learned octane number of the fuel.
At the present time there are various types of fuel that have different characteristics. These characteristics include the octane number. Among the various fuels, the octane number may commonly adopt the approximate values of 98, 95, 91 and 87. Fuels with octane numbers of 98 and 95 are often used in Western Europe, those with an octane number of 91 may be encountered in the United States and those with an octane number of 87 are used, for example, in Iran.
In order to optimize the compromise between performance, fuel consumption and reliability, it is desirable to have an engine setting, and notably the ignition advance setting, that is adapted to suit each octane number.
When the ignition advance setting is defined for one octane number and the fuel used has a different octane number, notably when changing geographical zone, the engine may operate unsatisfactorily. It may, for example, begin to ping, particularly if the engine is optimized for a fuel that has a higher octane number than the fuel being used, or alternatively the capability of the engine may not be best optimized if the fuel being used has an octane number lower than that for which the engine is set up.
Pinging may notably be due to a phenomenon of abnormal combustion involving detonation that notably generates a significant heat transfer liable to damage the combustion chamber. Pinging may arise somewhat randomly under certain operating conditions, notably when the octane number of the fuel is not suited to the engine setting.
Techniques for correcting pinging include the anti-pinging correction explained hereinbelow. This is essentially used when the discrepancies are small. This correction, which is curative, is unable satisfactorily to correct the effects of pinging when the difference in octane number is too great.
This known anti-pinging correction involves two types of action:                a rapid correction, also known as a fast loop (BR) correction, which greatly reduces the ignition advance, and        a slow correction, also known as a slow loop (BL) correction, which reduces the ignition advance less significantly.        
Thus, for example, when pinging is detected at a time t, the fast loop (BR) and the slow loop (BL) are activated in order to obtain a correction value of X° by which the ignition advance needs to be corrected in order to eliminate pinging.
At a time t+1, if pinging is no longer detected, the value of the slow loop (BL) is reduced by a given value. This value is reduced again each time t+1 at which pinging is not detected.
If the pinging phenomenon is detected again, the fast loop (BR) and the slow loop (BL) are activated again in order once again to correct the ignition advance.
A device capable of managing two settings based on two different octane numbers is also known. If pinging is detected over a certain period of time with a setting based on the octane number of 98, the setting is modified so that it becomes based on the setting for the octane number of 91. When the engine stops, the setting is “reset to zero”, which means to say that the setting reverts to the default setting (based on an octane number of 98).
These correction devices are not, however, ideal. Specifically, either they lack performance and finesse regarding the octane number detected, or they are always on the edge of detecting pinging, and this may lead to continued damage to the combustion chamber.